This invention relates to the electrical measurement art, and more particularly to a new and improved non-contacting voltage follower.
Electrostatic voltmeters measure electrostatic fields and potentials of surfaces without current flow and in a non-contacting manner. A probe or sensor converts an electrostatic field or surface potential to an a.c. voltage, the magnitude of which is proportional to the field or potential being measured. This conversion is accomplished by a capacitive modulation process wherein the capacitive coupling between a detector or electrode associated with the probe and a surface associated with the field or potential to be measured is modulated or varied at a fixed periodic rate, usually by mechanically vibrating the detector at such rate. A voltage difference existing between the surface and electrode will induce an a.c. voltage on the probe electrode which is processed by circuitry to give a measurement.
Heretofore, the bandwidth of electrostatic voltage followers has been limited to a function of the electrostatic detector operating or vibrating frequency. The detector usually is mechanically vibrated either in a plane parallel to the test surface to alternately expose and shield the detector relative to the surface in conjunction with an aperture in an interposed wall or it is vibrated in a plane perpendicular to the test surface. The bandwidth limitation is imposed because of the fact that the feedback which is employed in voltage followers of this type to drive the average detector voltage level back to the unknown potential being measured must assume a level which looks average or at least steady for several cycles of the detector modulating signal. According to the Nyquist Sampling Theorem, the bandwidth of a sampling system cannot exceed one-half the sampling rate. In present electrostatic voltage followers of the modulated detector type, the mechanical rate of modulation does not exceed 1-2 kilohertz which establishes the theoretical maximum bandwidth at 500 hertz to 1000 hertz.